The present invention relates to a magnetism detecting apparatus which detects an object to be detected based on a change in a magnetic flux corresponding to the permeability of the object. Further, the present invention relates to a magnetic material identifying apparatus which identifies a magnetic material of the object to be detected using the magnetism detecting apparatus.
Conventionally, magnetic material identifying apparatus includes a card reader. In recent years, a higher coercive force is given to a magnetic strip formed on a card to improve reliability of data recorded thereon; accordingly, both high coercive force cards and low coercive force cards are intermingled (circulated) in the market. Hence, card readers are required to comprise a function which allows recording of data at a sufficient current corresponding to the various coercive forces of cards.
Therefore, based on the correlation between a coercive force and a permeability of a magnetic material, an applicant of the present invention applied for a grant of patent on a differential magnetic sensor which identifies a coercive force of a card by detecting a change in a magnetic flux corresponding to a permeability of a magnetic strip of the card.
However, further invention is required to the above invention. According to the above invention, the coercive force of a card is identified by detecting a change in a magnetic flux corresponding to the permeability of a magnetic strip; such a magnetic flux corresponding to the permeability of a magnetic strip is affected by the distance between the magnetic strip and the magnetic sensor (clearance). As a result, the coercive force of a card may not be accurately identified when the distance between the magnetic strip and the magnetic sensor is not maintained constant. For example, when a card is warped, the distance between the magnetic strips and the magnetic sensor is not constant when the card is inserted into the card reader. Also when dust adheres to the surface having a magnetic strip, the output of the sensor tends to be lower than normal. Accordingly, the change in the magnetic flux in these circumstances differs from normal conditions such that a coercive force of the card may be mistakenly identified.
FIG. 6 shows the relationship of the output from a magnetic sensor with the distance between the magnetic strip of a card and the magnetic sensor. Symbols therein indicate as follows: (a)=clearance characteristics of a card (medium) with a coercive force of 300 (Oe); (b)=clearance characteristics of a card with a coercive force of 650 (Oe); and (c)=clearance characteristics of a card with a coercive force of 2750 (Oe). For example, if the distance between the magnetic strip and the magnetic sensor is 0.1 mm when reading the card with 300 (Oe) wherein the magnetic strip and the magnetic sensor are completely in contact with each other, the output from the magnetic sensor as the card is identified is almost identical to the output for a card with 650 (Oe). As a result, the card with 300 (Oe) is mistakenly identified as a card with 650 (Oe). Also, if the distance between the magnetic strip and the magnetic sensor is more than 0.37 mm when reading a card with 300 (Oe), the card may be mistaken with a card with 2750 (Oe).
To solve the above issue, the present invention intends to provide a magnetism detecting apparatus which accurately identifies the coercive force of a magnetic material, such as a magnetic strip, and a magnetic material identifying apparatus using the same.